It has been shown previously (Bozler 1977) that in skeletal muscle shortening is time and length independent during most of the rising phase of an isotonic twitch in contrast to cardiac muscle. The time course of shortening will be further analyzed by determining in detail the role of mechanical factors such as the non-contractile structures, and regulatory processes induced by shortening. A servosystem, which is almost completed, will be used to determine the effect of shortening on contractile activity by introducing length and force steps. The effect of fatigue and other conditions influencing the time course of twitches will be studied. In other experiments the effect of force on relaxation will be studied. Earlier work (Bozler 1975) has shown that in cardiac muscle the time course of isotonic relaxation does not vary with the load over a wide range of loads and is not modified by length changes under most conditions. This problem is important for the theory of crossbridge mechanics and will be further investigated under a variety of conditions. Of special interest also is the last phase of isotonic relaxation in skeletal muscles which lasts for more than a minute at 0 degrees and is absent under isometric conditions. Heat production during this phase will be determined.